Correction of accumulated errors in inertial measurement units attached to a user

ABSTRACT

A system including: a plurality of sensor modules having inertial measurement units and attached to different parts of a user (e.g., head, hands, arms) to measure their orientations; a plurality of optical marks attached to the user; a camera attached to the user; and a computing device configured to correct an accumulated error by detecting the optical marks in an image generated by the camera and identifying mismatches in directions of the optical marks (or the sensors, or parts of the users) as measured and/or calculated based on the image and the corresponding directions of the optical marks (or the sensors, or parts of the users) as measured and/or calculated from the orientation measurements generated by the sensor modules.

RELATED APPLICATIONS

The present application relates to U.S. patent application Ser. No. 15/847,669, filed Dec. 19, 2017 and entitled “Calibration of Inertial Measurement Units Attached to Arms of a User and to a Head Mounted Device,” U.S. patent application Ser. No. 15/817,646, filed Nov. 20, 2017 and entitled “Calibration of Inertial Measurement Units Attached to Arms of a User to Generate Inputs for Computer Systems,” U.S. patent application Ser. No. 15/813,813, filed Nov. 15, 2017 and entitled “Tracking Torso Orientation to Generate Inputs for Computer Systems,” U.S. patent application Ser. No. 15/792,255, filed Oct. 24, 2017 and entitled “Tracking Finger Movements to Generate Inputs for Computer Systems”, U.S. patent application Ser. No. 15/787,555, filed Oct. 18, 2017 and entitled “Tracking Arm Movements to Generate Inputs for Computer Systems”, and U.S. patent application Ser. No. 15/492,915, filed Apr. 20, 2017 and entitled “Devices for Controlling Computers based on Motions and Positions of Hands.” The entire disclosures of the above-referenced related applications are hereby incorporated herein by reference.

FIELD OF THE TECHNOLOGY

The embodiments disclosed herein relate to computer input devices in general and more particularly but not limited to input devices for virtual reality and/or augmented/mixed reality applications implemented using computing devices, such as mobile phones, smart watches, similar mobile devices, and/or other devices.

BACKGROUND

U.S. Pat. App. Pub. No. 2014/0028547 discloses a user control device having a combined inertial sensor to detect the movements of the device for pointing and selecting within a real or virtual three-dimensional space.

U.S. Pat. App. Pub. No. 2015/0277559 discloses a finger-ring-mounted touchscreen having a wireless transceiver that wirelessly transmits commands generated from events on the touchscreen.

U.S. Pat. App. Pub. No. 2015/0358543 discloses a motion capture device that has a plurality of inertial measurement units to measure the motion parameters of fingers and a palm of a user.

U.S. Pat. App. Pub. No. 2007/0050597 discloses a game controller having an acceleration sensor and a gyro sensor. U.S. Pat. No. D772,986 discloses the ornamental design for a wireless game controller.

Chinese Pat. App. Pub. No. 103226398 discloses data gloves that use micro-inertial sensor network technologies, where each micro-inertial sensor is an attitude and heading reference system, having a tri-axial micro-electromechanical system (MEMS) micro-gyroscope, a tri-axial micro-acceleration sensor and a tri-axial geomagnetic sensor which are packaged in a circuit board. U.S. Pat. App. Pub. No. 2014/0313022 and U.S. Pat. App. Pub. No. 2012/0025945 disclose other data gloves.

U.S. Pat. No. 9,600,925 and U.S. Pat. App. Pub. No. 2017/0053454 disclose calibration techniques for a virtual reality system using calibration data from an imaging device and calibration data from an inertial measurement unit on a virtual reality headset.

U.S. Pat. App. Pub. No. 2016/0378204 discloses a system to track a handheld device for a virtual reality environment using sensor data of the handheld device and the camera data of a head mounted display.

The disclosures of the above discussed patent documents are hereby incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements.

FIG. 1 illustrates a system to track movements of a user according to one embodiment.

FIGS. 2 and 3 illustrate examples user poses that may be combined to calibrate inertial measurement units attached to the arms of a user.

FIG. 4 illustrates a system to control computer operations according to one embodiment.

FIG. 5 illustrates a pose of a user to calibrate inertial measurement units attached to the arms of a user.

FIG. 6 illustrates a skeleton model for the calibration according to the user pose of FIG. 5.

FIG. 7 illustrates the computation of rotational corrections calibrated according to the user pose of FIG. 5.

FIG. 8 shows a method to calibrate the orientation of an inertial measurement unit according to one embodiment.

FIG. 9 shows a detailed method to perform calibration according to one embodiment.

FIG. 10 shows a skeleton model for the calibration of a head mounted device using the calibration pose of FIG. 5.

FIG. 11 illustrates the computation of a rotation for the calibration of the front facing direction of a head mounted device.

FIG. 12 illustrates the use of a camera for the calibration of the front facing direction of a head mounted device according to the user pose of FIG. 5.

FIG. 13 illustrates the determination of a direction from an image for the calibration of the front facing direction of a head mounted device.

FIG. 14 shows a method to calibrate the front facing direction of a head mounted device according to one embodiment.

FIG. 15 shows a detailed method to calibrate the front facing direction of a head mounted device according to one embodiment.

FIG. 16 illustrates the use of a head mounted camera for the direction measurements of optical marks on sensor modules.

FIG. 17 illustrates a scenario where the direction measurements of optical marks on sensor modules cannot be determined using a head mounted camera.

FIG. 18 illustrates a skeleton model for the correction of accumulated errors in inertial measurement units according to one embodiment.

FIG. 19 illustrates corrections made according to the model of FIG. 18.

FIGS. 20 and 21 illustrate the computation of the orientation of a forearm based on the orientations of the hand and the upper arm connected by the forearm.

FIG. 22 shows a method to correct accumulated errors in an inertial measurement unit according to one embodiment.

DETAILED DESCRIPTION

The following description and drawings are illustrative and are not to be construed as limiting. Numerous specific details are described to provide a thorough understanding. However, in certain instances, well known or conventional details are not described in order to avoid obscuring the description. References to one or an embodiment in the present disclosure are not necessarily references to the same embodiment; and, such references mean at least one.

At least some embodiments disclosed herein allow the determination of the deviation of the measurement space of a sensor device from a reference coordinate system using a convenient pose of a user wearing the sensor device on arms of the user.

FIG. 1 illustrates a system to track movements of a user according to one embodiment.

In FIG. 1, a user wears a number of sensor devices (111, 113, 115, 117 and 119). Each of the sensor devices (111-119) has an inertial measurement unit (IMU) to determine its orientation. Thus, the sensor devices (111-119) track the orientations of portions of user that are movable relative to the torso (101) of the user and relative to each other, such as the head (107), the upper arms (103 and 105), and the hands (106 and 108).

The sensor devices (111-119) communicate their movement/orientation measurements to a computing device (141), e.g., to control gesture input and/or an avatar of the user in a virtual reality, mixed reality, or augmented reality application in the computing device (141).

Typically, the measurements of the sensor devices (111-119) are calibrated for alignment with a common reference system, such as a coordinate system (100).

In FIG. 1, the coordinate system (100) is aligned with a number of directions relative to the user when the user is in the reference pose illustrated in FIG. 1. The direction Z is parallel to the vertical direction from the feet of the user to the head (107) of the user; the direction Y is parallel to the sideway direction from one hand (106) of the user to another hand (108) of the user; and the direction X is parallel to the front-back direction relative to the torso (101) of the user in the reference pose.

The directions X, Y and Z can be identified to the computing device (141) via combinations of reference poses, such as the poses illustrated in FIGS. 1-3.

When in the reference pose illustrated in FIG. 1, the arms (103 and 105) of the user extends horizontally in the sideways, in alignment with the direction Y of the coordinate system (100).

When in the reference pose illustrated in FIG. 2, the arms (103 and 105) of the user extends vertically in alignment with the direction Z of the coordinate system (100).

When in the reference pose illustrated in FIG. 3, the arms (103 and 105) of the user extends forwards and horizontally, in alignment with the direction X of the coordinate system (100).

From the pose of FIG. 1 to the pose of FIG. 2, the arms (103 and 105) of the user rotate along the direction X. Thus, by comparing the orientations of the sensor devices (113-119) that rotate with the arms (103 and 105) along the direction X from the pose of FIG. 1 to the pose of FIG. 2, the computing device (141) computes the direction X in the measurement spaces of the sensor devices (113-119).

From the pose of FIG. 2 to the pose of FIG. 3, the arms (103 and 105) of the user rotate along the direction Y. Thus, by comparing the orientations of the sensor devices (113-119) that rotate with the arms (103 and 105) along the direction Y from the pose of FIG. 2 to the pose of FIG. 3, the computing device (141) computes the direction Y in the measurement spaces of the sensor devices (113-119).

From the pose of FIG. 3 to the pose of FIG. 1, the arms (103 and 105) of the user rotate along the direction Z. Thus, by comparing the orientations of the sensor devices (113-119) that rotate with the arms (103 and 105) along the direction Z from the pose of FIG. 3 to the pose of FIG. 1, the computing device (141) computes the direction Z in the measurement spaces of the sensor devices (113-119).

Further, the direction Z can be computed as a direction orthogonal to the directions X and Y in a three dimensional space; the direction Y can be computed as a direction orthogonal to the directions X and Z in the three dimensional space; and the direction X can be computed as a direction orthogonal to the directions Y and Z in the three dimensional space.

Thus, from the measured orientations of the sensor devices (113-119) in the three reference poses illustrated in FIGS. 1-3, the computing device computes the relative orientation change between the reference coordinate system (100) and the measurement space of each of the sensor devices (113-119). Subsequently, the relative rotation can be used to calibration the corresponding one of the sensor devices (113-119) such that the calibrated orientation measurements relative to the common reference coordinate system (100).

The head sensor (111) can also be calibrated to produce measurements relative to the common reference coordinate system (100). For example, the average of the orientations of the head sensor (111) in the reference poses of FIGS. 1-3, or one of the orientations, can be considered as a reference orientation that is aligned with the common reference coordinate system (100); and the subsequent measurements of the head sensor (111) can be calibrated as being relative to the reference orientation.

After the measurements of the sensor devices (111-119) are calibrated to measure relative to the common reference coordinate system (100), the hands, arms (105, 103), the head (107) and the torso (101) of the user may move relative to each other and relative to the coordinate system (100). The measurements of the sensor devices (111-119) provide orientations of the hands (106 and 108), the upper arms (105, 103), and the head (107) of the user relative to the common coordinate system (100).

In some implementations, each of the sensor devices (111-119) communicates its measurements directly to the computing device (141) in a way independent from the operations of other sensor devices.

Alternative, one of the sensor devices (111-119) may function as a base unit that receives measurements from one or more other sensor devices and transmit the bundled and/or combined measurements to the computing device (141).

Preferably, wireless connections made via a personal area wireless network (e.g., Bluetooth connections), or a local area wireless network (e.g., Wi-Fi connections) are used to facilitate the communication from the sensor devices (111-119) to the computing device (141).

Alternatively, wired connections can be are used to facilitate the communication among some of the sensor devices (111-119) and/or with the computing device (141).

For example, a hand module (117 or 119) attached to or held in a corresponding hand (106 or 108) of the user may receive the motion measurements of a corresponding arm module (115 or 113) and transmit the motion measurements of the corresponding hand (106 or 108) and the corresponding upper arm (105 or 103) to the computing device (141).

For example, the hand modules (117 and 119) and the arm modules (115 and 113) can be each respectively implemented via a base unit (or a game controller) and an arm/shoulder module discussed in U.S. patent application Ser. No. 15/492,915, filed Apr. 20, 2017 and entitled “Devices for Controlling Computers based on Motions and Positions of Hands”, the entire disclosure of which application is hereby incorporated herein by reference.

In some implementations, the head module (111) is configured as a base unit that receives the motion measurements from the hand modules (117 and 119) and the arm modules (115 and 113) and bundles the measurement data for transmission to the computing device (141). In some instances, the computing device (141) is implemented as part of the head module (111).

The head module (111) may further determine the orientation of the torso (101) from the orientation of the arm modules (115 and 113) and/or the orientation of the head module (111) and thus eliminates the need to attach a separate sensor device to the torso (101), as discussed in U.S. patent application Ser. No. 15/813,813, filed Nov. 15, 2017, the disclosure of which is hereby incorporated herein by reference.

For the determination of the orientation of the torso (101), the hand modules (117 and 119) are optional in the system illustrated in FIG. 1. Further, in some instances the head module (111) is not used, in the tracking of the orientation of the torso (101) of the user.

Optionally, the computing device (141), or a hand module (e.g., 117), may combine the measurements of the hand module (e.g., 117) and the measurements of the corresponding arm module (e.g., 115) to compute the orientation of the forearm connected between the hand (106) and the upper arm (105), in a way as disclosed in U.S. patent application Ser. No. 15/787,555, filed Oct. 18, 2017 and entitled “Tracking Arm Movements to Generate Inputs for Computer Systems”, the entire disclosure of which is hereby incorporated herein by reference.

FIG. 4 illustrates a system to control computer operations according to one embodiment. For example, the system of FIG. 4 can be implemented via attaching the arm modules (115, 113) to the upper arms (105 and 103) respectively and optionally, the head module (111) to the head (107), in a way illustrated in FIGS. 1-3.

In FIG. 4, the head module (111) and the arm module (113) have micro-electromechanical system (MEMS) inertial measurement units (IMUs) (121 and 131) that measure motion parameters and determine orientations of the head (107) and the upper arm (103). Similarly, the hand module (119) may also have its IMU.

Each of the IMUs (131, 121) has a collection of sensor components that enable the determination of the movement, position and/or orientation of the respective IMU along a number of axes. Examples of the components are: a MEMS accelerometer that measures the projection of acceleration (the difference between the true acceleration of an object and the gravitational acceleration); a MEMS gyroscope that measures angular velocities; and a magnetometer that measures the magnitude and direction of a magnetic field at a certain point in space. In some embodiments, the IMUs use a combination of sensors in three and two axes (e.g., without a magnetometer).

The computing device (141) has a motion processor (145), which includes a skeleton model (143) of the user (e.g., illustrated FIG. 6). The motion processor (145) controls the movements of the corresponding parts of the skeleton model (143) according to the movements/orientations of the upper arms (103 and 105) measured by the arm modules (113 and 115), the movements/orientation of the head (107) measured by the head module (111), the movements/orientations of the hand (106 and 108) measured by the hand modules (117 and 119), etc.

The skeleton model (143) is controlled by the motion processor (145) to generate inputs for an application (147) running in the computing device (141). For example, the skeleton model (143) can be used to control the movement of an avatar/model of the arms (105 and 103), the hands (106 and 108), the head (107), and the torso (101) of the user of the computing device (141) in a video game, a virtual reality, a mixed reality, or augmented reality, etc.

Preferably, the arm module (113) has a microcontroller (139) to process the sensor signals from the IMU (131) of the arm module (113) and a communication module (133) to transmit the motion/orientation parameters of the arm module (113) to the computing device (141). Similarly, the head module (111) has a microcontroller (129) to process the sensor signals from the IMU (121) of the head module (111) and a communication module (123) to transmit the motion/orientation parameters of the head module (111) to the computing device (141).

Optionally, the arm module (113) and the head module (111) have LED indicators (137 and 127) respectively to indicate the operating status of the modules (113 and 111).

Optionally, the arm module (113) has a haptic actuator (138) respectively to provide haptic feedback to the user.

Optionally, the head module (111) has a display device (127) and/or buttons and other input devices (125), such as a touch sensor, a microphone, a camera, etc.

In some implementations, the head module (111) is replaced with a module that is similar to the arm module (113) and that is attached to the head (107) via a strap or is secured to a head mounted display device.

In some applications, the hand module (119) can be implemented with a module that is similar to the arm module (113) and attached to the hand via holding or via a strap. Optionally, the hand module (119) has buttons and other input devices, such as a touch sensor, a joystick, etc.

For example, the handheld modules disclosed in U.S. patent application Ser. No. 15/792,255, filed Oct. 24, 2017 and entitled “Tracking Finger Movements to Generate Inputs for Computer Systems”, U.S. patent application Ser. No. 15/787,555, filed Oct. 18, 2017 and entitled “Tracking Arm Movements to Generate Inputs for Computer Systems”, and/or U.S. patent application Ser. No. 15/492,915, filed Apr. 20, 2017 and entitled “Devices for Controlling Computers based on Motions and Positions of Hands” can be used to implement the hand modules (117 and 119), the entire disclosures of which applications are hereby incorporated herein by reference.

FIG. 4 shows a hand module (119) and an arm module (113) as examples. An application for the tracking of the orientation of the torso (101) typically uses two arm modules (113 and 115) as illustrated in FIG. 1. The head module (111) can be used optionally to further improve the tracking of the orientation of the torso (101). Hand modules (117 and 119) can be further used to provide additional inputs and/or for the prediction/calculation of the orientations of the forearms of the user.

Typically, an IMU (e.g., 131 or 121) in a module (e.g., 113 or 111) generates acceleration data from accelerometers, angular velocity data from gyrometers/gyroscopes, and/or orientation data from magnetometers. The microcontrollers (139 and 129) perform preprocessing tasks, such as filtering the sensor data (e.g., blocking sensors that are not used in a specific application), applying calibration data (e.g., to correct the average accumulated error computed by the computing device (141)), transforming motion/position/orientation data in three axes into a quaternion, and packaging the preprocessed results into data packets (e.g., using a data compression technique) for transmitting to the host computing device (141) with a reduced bandwidth requirement and/or communication time.

Each of the microcontrollers (129, 139) may include a memory storing instructions controlling the operations of the respective microcontroller (129 or 139) to perform primary processing of the sensor data from the IMU (121, 131) and control the operations of the communication module (123, 133), and/or other components, such as the LED indicator (137), the haptic actuator (138), buttons and other input devices (125), the display device (127), etc.

The computing device (141) may include one or more microprocessors and a memory storing instructions to implement the motion processor (145). The motion processor (145) may also be implemented via hardware, such as Application-Specific Integrated Circuit (ASIC) or Field-Programmable Gate Array (FPGA).

In some instances, one of the modules (111, 113, 115, 117, and/or 119) is configured as a primary input device; and the other module is configured as a secondary input device that is connected to the computing device (141) via the primary input device. A secondary input device may use the microprocessor of its connected primary input device to perform some of the preprocessing tasks. A module that communicates directly to the computing device (141) is consider a primary input device, even when the module does not have a secondary input device that is connected to the computing device via the primary input device.

In some instances, the computing device (141) specifies the types of input data requested, and the conditions and/or frequency of the input data; and the modules (111, 113, 115, 117, and/or 119) report the requested input data under the conditions and/or according to the frequency specified by the computing device (141). Different reporting frequencies can be specified for different types of input data (e.g., accelerometer measurements, gyroscope/gyrometer measurements, magnetometer measurements, position, orientation, velocity).

In general, the computing device (141) may be a data processing system, such as a mobile phone, a desktop computer, a laptop computer, a head mounted virtual reality display, a personal medial player, a tablet computer, etc.

After the initial calibration of the sensor devices (111-119) for alignment with the common coordinate system (100), the sensor devices (113-119) may move relative to the user. For example, the arm module (e.g., 115) may slip around the arm (105) of the user after a game in virtual reality or augmented reality implemented via the computing device (141). The sensor devices mounted on the arms of the user can be conveniently re-calibrated using a reference pose illustrated in FIG. 5.

FIG. 5 illustrates a pose of a user to calibrate inertial measurement units attached to the arms of a user. For example, the calibration pose of FIG. 5 can be used for recalibration after the sensor devices (111-119) are initially calibrated according to the reference poses of FIGS. 1-3 using the computing system illustrated in FIG. 4.

In FIG. 5, the user extends arms (103 and 105) and hands (106 and 108) straight in a horizontal plane XY. The hand modules (117 and 119) are in contact with each other. The arms (103 and 105) form small angles respectively with the front facing direction X. The left and right arms (103 and 105) and the left and right hands (106 and 108) are in positions symmetric with respect to the center alone the front facing direction X. The front facing direction of the torso (101) of the user aligns with the direction X. In some instances, the calibration pose of FIG. 5 defines an updated coordinate system (100) that may have rotated along the vertical axis Z by an angle from the initial coordinate system (100) at the time of the calibration pose of FIG. 1.

The orientations of the arm modules (103 and 105) at the pose of FIG. 5 can be used to calibrate the arm modules (103 and 105) for alignment with the common coordinate system (100), as further discussed below. The calibration corrects at least the twist or turn of the arm module (e.g., 115) around the arm (105) between the time of the initial calibration (e.g., calibrated according to the reference pose of FIG. 1) and the time of recalibration using the pose of FIG. 5.

Further, the direction V_(A) (159) of the arm (105) in the updated coordinate system (100) aligned with the front facing direction of the torso (101) at the calibration pose of FIG. 5 can be computed from the shoulder width of the user, the arm length of the user, and the width of the hand module (117) and used to calibrate the orienation of the arm module (115) within the horizontal plane XY.

FIG. 6 illustrates a skeleton model for the calibration according to the user pose of FIG. 5. For example, the skeleton model of FIG. 6 can be used to compute the direction V_(A) (159) of the arm (105) for calibration in the horizontal plane XY.

In FIG. 6, the skeleton model has a known shoulder width L_(S) (155), a known arm length_(S) (153), and a known width W (151) of the handheld device (117). Thus, the distance L (157) between the handheld device (117) and the torso (101), as well as the direction V_(A) (159) of the arm (105) relative to the updated coordinate system (100), can be computed from the shoulder width L_(S) (155), the arm length_(S) (153), and the width of the handheld device (117).

The skeleton model of FIG. 6 illustrates an arm (e.g., 105) as one rigid unit. Alternatively, other skeleton models may include separate forearms and upper arms that are connected via elbow joints and that may move relative to each other. Further, a skeleton model may include the head (107) and/or other portions of the user.

FIG. 7 illustrates the computation of rotational corrections calibrated according to the user pose of FIG. 5. For example, the computation of FIG. 7 can be performed in the computing system of FIG. 4 using the calibration pose of FIG. 5 and the skeleton model of FIG. 6, after the sensor devices (111-119) are initially calibrated for alignment with a common coordinate system (100) using the reference poses of FIGS. 1-3.

FIG. 7 illustrates a rotation γ (165) along the lengthwise direction of the arm (105) while the arm (105) is at the reference pose of FIG. 1 and a rotation τ within the horizontal plane XY along the direction Z.

The orientation change of the arm module (115) with respect to the common reference system (100) can be considered as the result of a rotation γ (165) around the arm (105) while the arm (105) is pointing along the direction Y and then a rotation (163) within the horizontal plane XY along the direction Z.

Thus, the rotation γ (165) can be computed by removing the rotation from the orientation change of the arm module (115) with respect to the common reference system (100).

For example, the rotational transformation of the arm module (115) as measured by the arm module (115) relative to the common reference system (100) can be applied, as a rotation α (161), to a unit direction that is initially along the direction Y to obtain the lengthwise direct R (171) of the arm (105) at the time of the calibration pose of FIG. 5. The projection of the lengthwise direct R (171) at the time of the calibration pose of FIG. 5, projected onto the horizontal plane XY along the direction Z, provides a direction R_(h) that has a rotation β (163) within the horizontal plane XY to the direction Y. The angle of the rotation β (163) can be calculated as the rotation between the direction Y and the direction R_(h). Reversing the rotation β (163) from the rotational transformation of the arm module (115) from the reference coordinate system (100) to the calibration pose of FIG. 5, as measured by the arm module (115), provides the rotation γ (165).

When the user moves the arm (105) from the reference pose of FIG. 1 to the calibration pose of FIG. 5, the arm (105) rotates along its lengthwise direction by a predetermined angle (e.g., 39 degrees). Thus, the difference between the calculated rotation γ (165) and the predetermined angle (e.g., 39 degrees) provides a desired correction (e.g., for a result of the arm module (115) slipping around the arm (105) after the initial calibration).

After calibrating the measurement of the arm module (115) to account for the slipping of the arm module (115) around the arm (105), it may be desirable to further recalibrate to account for the rotation of the torso (101) along the vertical direction, such that the front facing direction X as calibrated is aligned with the front facing direction of the torso (101) at the time of the calibration pose of FIG. 5.

Since the arm (105) is expected to be in the direction V_(A) (159), the rotation τ (167), rotating in the horizontal plane XY along the direction Z from the direction R_(h) (173) to the direction V_(A) (159), can be applied to the orientation measurement of the arm module (115) such that the measurements of the arm module (115) is calibrated according to the new front facing direction identified by the torso (101) at the time of the calibration pose of FIG. 5.

The computation of the calibration parameters (e.g., γ (165) and τ (167)) can be performed in the computing device (141). Subsequently, the computing device (141) may apply the calibration parameters to the measurement results received from the sensor device (e.g., 115).

Alternatively, the computing device (141) may transmit the calibration parameters (e.g., γ (165) and τ (167)) to the sensor device (e.g., 115) that calibrates subsequent measurements relative to the common coordinate system (100) as defined by the calibration pose of FIG. 5.

In some implementations, the computing device (141) transmits the parameters used for the calibration calculation to the sensor device (e.g., 115), such as the direction V_(A) in the new coordinate system (100) aligned with the calibration pose of FIG. 5, and/or the predetermined angle of rotation (e.g., 39 degrees) along the lengthwise direction when the arm (e.g., 105) is rotated from the reference pose of FIG. 1 to the calibration pose of FIG. 5. The sensor device (e.g., 115) calculates the calibration parameters (e.g., γ (165) and τ (167)) from its current orientation measurement upon an indication from the computing device (141) that the user is in the calibration pose of FIG. 5.

FIG. 8 shows a method to calibrate the orientation of an inertial measurement unit according to one embodiment. For example, the method of FIG. 8 can be implemented in the computing system of FIG. 4 using a calibration pose of FIG. 5, the skeleton model of FIG. 6, and the rotation relations illustrated in FIG. 7, after a sensor device is calibrated using the reference poses of FIGS. 1-3.

The method of FIG. 8 includes: receiving (201) an measurement of the orientation of a sensor module (115) attached to an arm (105) of a user in a calibration pose (e.g., illustrated in FIG. 5) relative to a reference pose (e.g., illustrated in FIG. 1); calculating (203) a first rotation γ (165) of the sensor module (115) around a lengthwise direction of the arm (105) from the reference pose (e.g., illustrated in FIG. 1) to the calibration pose (e.g., illustrated in FIG. 5); calculating (205) a second rotation τ (167) of the sensor module (115) around a vertical axis Z, from a projection R_(h) (173) of a lengthwise direction R (171) of the arm (115) at the calibration pose (e.g., illustrated in FIG. 5), projected in a horizontal plane XY, to an expected lengthwise direction V_(A) (159) of the arm (105) in an updated reference pose (e.g., a pose obtained by rotating the arm (105) in the horizontal plane XY to the side, in parallel with the torso (101), to be in a pose similar to that illustrated in FIG. 1, without rotating the torso (101)); and calibrating (207) subsequent orientation measurements of the sensor module (115) using the first rotation γ (165) and the second rotation τ (167).

From the time of initial calibration with respect to the reference pose (e.g., illustrated in FIG. 1) and the time of recalibration at the calibration pose (e.g., illustrated in FIG. 5), the front facing direction of the torso (101) may have shifted by rotating along the vertical axis Z. Further, the sensor module (115) attached to the arm (105) via an armband may have slipped around the arm (105) about the lengthwise direction of the arm (105). The recalibration realigns the orientation measurement of the sensor module (115) with an updated reference coordinate system (101) that is aligned with respect to the update reference pose, without requiring the user to make the update reference pose for the recalibration. The update reference pose corresponds to the pose obtained by the user rotating the arm (105) from the calibration pose illustrated in FIG. 5 to a position where the lengthwise direction is in parallel with the torso (101) and is perpendicular to the front facing direction of the torso (101). The recalibration is performed relative to the update reference pose without requiring the user to actually make the update reference pose.

When the arm (105) rotates from the reference pose (e.g., illustrated in FIG. 1) to the calibration pose (e.g., illustrated in FIG. 5), the arm (105) rotates about its lengthwise direction by a predetermined angle (e.g., 39 degrees). The difference between the first rotation γ (165) and the rotation of the predetermined angle (e.g., 39 degrees) represents the slip of the sensor module (115) around the arm (105) from the time of the calibration according to the reference pose (e.g., illustrated in FIG. 1) and the time of the recalibration using the calibration pose (e.g., illustrated in FIG. 5). The slip can be removed from subsequent measurements. The second rotation τ (167) can be applied to the subsequent measurements so that the subsequent measurements are calibrated according to the front facing direction of the torso (101) at the time of the calibration pose (e.g., illustrated in FIG. 5).

FIG. 9 shows a detailed method to perform calibration according to one embodiment. For example, the method of FIG. 9 can be implemented in the computing system of FIG. 4 using a calibration pose of FIG. 5, the skeleton model of FIG. 6 and the rotation relations illustrated in FIG. 7, after a sensor device is calibrated using the reference poses of FIGS. 1-3.

The method of FIG. 9 includes: rotating (221) a lengthwise direction (e.g., parallel to the direction Y in FIG. 1 or 7) of an arm (105) from a reference pose (e.g., illustrated in FIG. 1) to a calibration pose (e.g., illustrated in FIG. 5) according to an orientation measurement from a sensor device (115) attached to the arm (105). The arm (105) is in a horizontal plane XY both in the reference pose (e.g., illustrated in FIG. 1) and in the calibration pose (e.g., illustrated in FIG. 5).

After the rotation α (161) according to the orientation measurement, the lengthwise direction of the arm (105) arrives at the direction R (171) illustrated in FIG. 7. The method of FIG. 9 further includes: projecting (223) the lengthwise direction R (171) onto the horizontal plane XY to obtain the direction R_(h) (173) illustrated in FIG. 7; and calculating (225) a rotation β (163), from the lengthwise direction (e.g., parallel to the direction Y in FIG. 1 or 7) of the arm (105) at the reference pose (illustrated in FIG. 1), to the lengthwise direction R_(h) (173) as projected onto the horizontal plane XY.

The method of FIG. 9 further includes: removing (227) the rotation β (163) from an orientation change of the sensor device (115) from the reference pose (e.g., illustrated in FIG. 1) to the calibration pose (e.g., illustrated in FIG. 5) to obtain a rotation γ (165) along the lengthwise direction of the arm (105); and deducting (229), from the rotation γ (165) along the lengthwise direction of the arm (105), a predetermined rotation of the arm (105) about its lengthwise direction from the reference pose (e.g., illustrated in FIG. 1) to the calibration pose (e.g., illustrated in FIG. 5) to obtain a twist of the sensor device (115) around the arm (e.g., from the time of the initial calibration according to the reference pose to the time of the calibration pose).

Optionally, the method of FIG. 9 further includes: calculating (231) a rotation τ (167), from the lengthwise direction R_(h) (173) as projected onto the horizontal plane XY, to a predetermined direction V_(A) (159) of the arm (105) in an updated reference pose (e.g., corresponding to the calibration pose of FIG. 5, by changing to a pose similar to the calibration pose of FIG. 1 through rotating the arms (103 and 105) without moving the torso (101)) to determine a rotation τ (167) of a front facing direction Y of the user from the time of the reference pose (illustrated in FIG. 1) to the time of the calibration pose (illustrated in FIG. 5).

The method of FIG. 9 further includes calibrating (233) subsequent orientation measurements of the sensor device (115) relative to the updated reference pose by removing the twist and optionally applying the rotation τ (167) of the front facing direction of the user.

The methods of FIGS. 8 and 9 can be implemented in the computing device (141) separate from the arm module (115), or implemented in the arm module (115). In some instances, the calibration parameters are calculated by the computing device (141) can be communicated to the arm module (115) for combination with subsequent measurements. In other implementations, a handheld device functions as a primary input device that calculates the calibration parameters and/or performs the calibration for the arm module (115) that functions as a secondary input device.

When the user is in the calibration pose illustrated in FIG. 5, it is also desirable to calibrate the orientation measurements of the head module (111).

FIG. 10 shows a skeleton model for the calibration of a head mounted device using the calibration pose of FIG. 5.

In FIG. 10, the arms (e.g., 103) of the user are in a horizontal plane XY. The head module (111) is above the horizontal plane containing the arms (e.g., 103) of the user and may or may not be aligned with the front facing direction X in general.

In one method for the calibration of the orientation measurements of the head module (111), a user is required to position the head module (111) that is mounted on the head (107) of the user such that the front facing direction of the head mounted device (111) is parallel with or in alignment with the global front facing direction X as defined by the directions of the arms (e.g., 103 and 105, as illustrated in FIGS. 5 and 6).

For example, the user may be instructed to look straight ahead horizontally in a horizontal plane such that the front facing direction of the head mounted device (111) is the same as the global front facing direction X at the calibration pose of FIG. 5. When the head module (111) is positioned in such a way at the calibration pose, the orientation measurement of the head mounted device (111) at the calibration pose relative to the reference pose (e.g., illustrated in FIG. 1) can be used to recalibrate the subsequent orientation measurements of the had module (111) according to the global reference system XYZ at the calibration pose of FIG. 5. Thus, the head module (111) and the arm modules (113 and 115) are all calibrated according to the same global reference system XYZ at the calibration pose of FIG. 5.

FIG. 11 illustrates the computation of a rotation for the calibration of the front facing direction of a head mounted device.

In FIG. 11, the front facing direction X at the global coordinate system of the reference system is rotated (305) to the direction H (301) according to the orientation measurement of the head module (111) at the calibration pose. Specifically, the orientation measurement of the head module (111) at the calibration pose relative to the reference pose defines a rotation α (305) that rotates the front facing direction X in the reference pose of FIG. 1 to the front facing direction H (301) in the calibration pose of FIG. 5.

To perform the calibration, the direction H (301) is projected on to the horizontal plane XY to obtain the projected direction H_(h) (303).

From the projected direction H_(h) (303), a rotation β (307) along the vertical direction Z from the front facing direction X in the reference pose of FIG. 1 to the projected direction H_(h) (303) can be calculated for the calibration of subsequent orientation measurements of the head module (111). For example, when the rotation (307) is subtracted from the subsequent orientation measurements of the head module (111) for calibration, the calibrated orientation measurement is relative to the global front facing direction X at the calibration pose.

When the head module (111) includes a camera (or when the camera is attached to the head module (111), which may include a head mounted display (HMD)), a direction X_(I) (323) of the head module (111) that is in alignment with the global front facing direction X at the calibration pose can be calculated from an image captured using the camera, as further discussed below. When the direction X_(I) (323) can be determined using the camera, the user does not have to look straight ahead in a horizontal plane to position the head module (111) in such a way that the front facing direction of the head mounted device (111) is in alignment with the global front facing direction X. Since the direction X_(I) (323) is in alignment with the global front facing direction X, the rotational transformation of the head module (111) from the reference pose to the calibration pose can be applied to the direction X_(I) (323) to obtain the direction H (301) that faces the front direction. It's projection direction H_(h) (303) in the horizontal plane defines the rotation β (307) from the direction X. Thus, the correctional rotation β (307) can be determined with the use of the camera without requiring the user to position the head module (111) in a precise orientation where the front facing direction of the head module (111) is in alignment with the global front facing direction X at the calibration pose (at least in the vertical plane that contains the global front facing direction X at the calibration pose).

FIG. 12 illustrates the use of a camera for the calibration of the front facing direction of a head mounted device according to the user pose of FIG. 5.

In FIG. 12, each of the handheld devices (117 and 119) has an optical mark (312 and 314) that can be captured and identified in an automated way in an image generated by a camera (309) (e.g., a camera included in the head module (111) or a camera attached to the head module (111), such as a head mounted display).

For example, the optical marks (312 and 314) may be implemented using Light-Emitting Diode (LED) lights (e.g., visible lights or infrared lights detectable via the camera (309)) installed on the handheld devices. The light-Emitting Diode (LED) lights are switched on when the handheld devices (117 and 119) are in use and/or when the user is in the calibration pose of FIG. 12 (or FIG. 5).

For example, the handheld devices (117 and 119) may include one or more buttons which, when activated, indicate that the user is in the calibration pose, which cause the light-Emitting Diode (LED) lights to be on and cause the camera (309) to capture an image for calibration.

For example, being in the calibration pose of FIG. 12 (or FIG. 5) for a time period longer than a threshold can be identified as a gesture for calibration. When the gesture is detected from the sensor modules (e.g., 113, 115, 117, and 119), the light-Emitting Diode (LED) lights are turned on to simplify the detection of their locations in the image(s) captured by the camera (309).

In some instances, the optical marks (312 and 314) may be inactive elements, such as decorative marks made of paints, stickers, a geometrical feature of the house of the handheld devices (117 and 119). In some instances, a pattern analysis of the captured image of the handheld devices (117 and 119) and/or the hands (106 and 108) of the user is performed to recognize the handheld devices (117 and 119) and/or the hands (106 and 108) and thus their locations in the image.

FIG. 12 illustrates the locations (313 and 315) of the optical marks (312 and 314) on the image (317) captured by the camera (309). A center point O can be identified from the optical marks (312 and 314). The direction V_(O) (311) of the center point O relative to the camera (309) can be calculated from the field of view (319) of the camera, the relative position of the center point O on the image (317).

In general, the direction V_(O) is not in parallel with the front facing direction of the camera (309) and is not in parallel with the front facing direction X of the global coordinate system XYZ (100) at the calibration pose of FIG. 12.

FIG. 12 illustrates an orientation of the camera (309), where the front facing direction of the head module (111) and the camera (309) is in parallel with (aligned with) the front facing direction X of the global coordinate system XYZ (100) (e.g., when the user looks straight ahead in the horizontal plane). In such a situation, the horizontal rotation of the front facing direction of the head module (111) and the camera (309) from the reference pose to the calibration pose identifies the rotational correction for the head module (111). For example, the orientation measurement of the head module (111) at the calibration pose provides a rotation transformation (305) from the reference pose to the calibration pose. When the rotation transformation (305) is applied to the front facing direction at the reference pose (e.g., illustrated in FIG. 1) to obtain the direction H (301) as illustrated in FIG. 11, its projection H_(h) (303) defines a rotate β (307) from the front facing direction X at the calibration pose, which can be used to re-calibrate the subsequent orientation measurements relative to the front facing direction X of the global coordinate system (100) at the calibration pose of FIG. 12, as discussed above in connection with FIG. 11.

In general, the user may not look straight ahead in the horizontal plane. Thus, the center point of the optical marks (312 and 314) captured in the image (317) in general may not be at point O. Instead of at the direction V_(O) (311), the center point of the optical marks (312 and 314) captured in the image (317) in general is at a direction V_(I) (321) relative to the camera (309) (as illustrated in FIG. 13). The deviation of the direction V_(I) (321) from the direction V_(O) (311) can be used to identify a direction X_(I) (323) that is in alignment with the front facing direction X of the global coordinate system (100) at the calibration pose of FIG. 12. In general, the direction X_(I) (323) is not in alignment with the front facing direction of the camera (309). After the direction X_(I) (323) is determined from the image (317) captured by the camera (309), applying the rotation transformation between the reference pose and the calibration pose to the direction X_(I) (323) provides the direction H (301) illustrated in FIG. 11, the rotation correction β (307) from the front facing direction X at the reference pose can be determined from its projection H_(h) (303) in the horizontal plan XY, as illustrated in FIG. 11, without requiring the user to look straight ahead in the horizontal plane.

FIG. 13 illustrates the determination of a direction X_(I) (323) from a camera image captured at a calibration pose of FIG. 12 or FIG. 5 for the calibration of the front facing direction of a head mounted device (111).

In FIG. 13, when the front facing direction x of the camera (309) and the head module (111) is aligned with the global front facing direction X (e.g., as in FIG. 12), the direction V_(O) (311) of the center O of the optical marks (312, 314) captured by the camera (309) in the image (317) has a rotational offset ω (322) from the front facing direction x (e.g., as in FIG. 12).

When the camera (309) is rotated to capture the center O of the optical marks (312, 314) at the direction V_(I) (321), the direction X_(I) (323) is in alignment with the global front facing direction X. As illustrated in FIG. 13, the direction X_(I) (323) has the same rotational offset ω (322/324) from the direction V_(I) (321). Thus, the direction X_(I) (323) can be calculated by applying the rotational offset ω (322/324) to the direction V_(I) (321). Alternatively, the rotation (326) from the direction V_(O) (311) to the direction V_(I) (321) can be applied (325) to the front facing direction x of the camera (309) to obtain the direction X_(I) (323).

FIG. 13 illustrates the camera rotation (325 or 326) in the horizontal plane where the user at the calibration pose looks in the horizontal plane but does not look exactly straight ahead. In general, the user may also look down at the optical marks (312 and 314) and thus does not look in the horizontal plane. In such a situation the camera rotation from the direction V_(O) to the direction V_(I) may include a rotation out of the horizontal plane. The determination of the direction X_(I) (323) can be performed by applying the rotational offset ω (322) to the direction V_(I) (321), or applying the rotation (326) to the front facing direction x, in the same way as illustrated in FIG. 13.

The offset ω (322) can be calculated from the coordinate offsets of the center point O in the local coordinate system of the camera (309) at a time the local coordinate system of the camera (309) is aligned with the global coordinate system XYZ (100) as illustrated in FIG. 12. For example, the coordinate offsets include the vertical offset between the camera (309) and the horizontal plane that goes through the center O at the calibration pose, the lateral offset between the camera (309) and a vertical plane that goes through the center of the user at the calibration pose and the center O, and the frontal offset between the camera (309) and a vertical plane that is perpendicular to the global front facing direction X and goes through the center O.

Since the direction X_(I) (323) is aligned with the direction X of the global coordinate system XYZ (100) at the time of the calibration pose (e.g., as illustrated in FIG. 5 or 12), the orientation transformation measured at the calibration pose relative to the reference pose (e.g., as illustrated in FIG. 1) can be applied to the direction X_(I) (323) at the reference pose to obtain the direction H (301) illustrated in FIG. 11. A correction according to the rotation β (307) between the direction X and the projection H_(h) (303) as illustrated in FIG. 11 can be applied to the subsequent orientation measurements of the head module (111) such that the calibrated measurements are relative to the global coordinate system XYZ (100) defined at the time of the calibration pose (e.g., illustrated in FIG. 5 or 15).

FIG. 14 shows a method to calibrate the front facing direction of a head mounted device according to one embodiment. For example, the method of FIG. 14 can be implemented in a system illustrated in FIG. 4, using a calibration pose illustrated in FIG. 5 or 12 relative to a reference pose illustrated in FIG. 1 and a skeleton model of FIG. 10, in accordance with the rotational offset illustrated in FIG. 13.

The method of FIG. 14 includes: receiving (331) an measurement of the orientation of a sensor module (111) attached to a head mounted device (111) of a user in a calibration pose (e.g., illustrated in FIG. 5 or FIG. 12) relative to a reference pose (e.g., illustrated in FIG. 1); identifying (333) a first direction X_(I) (323) of the sensor module (111) in the reference pose (e.g., illustrated in FIG. 1) that is expected to be rotated to a front facing direction X of the global coordinate system (100) at the calibration pose (e.g., illustrated in FIG. 5 or FIG. 12); applying (335) a first rotation α (305), identified by the measurement of the orientation of the sensor module (111), to the first direction X_(I) (323) to obtain a second direction H (301) of the sensor module (121) at the calibration pose (e.g., illustrated in FIG. 5 or 12) relative to the reference pose (e.g., illustrated in FIG. 1); determining (337) a second rotation β (307) between the front facing direction X at the reference pose (e.g., illustrated in FIG. 1) and a projection H_(h) (303) of the second direction H (301) in a horizontal plane XY; and calibrating (339) subsequent orientation measurements of the sensor module (121) using the second rotation β (307).

When the user is instructed to look straight ahead in the horizontal plane at the calibration pose (e.g. as illustrated in FIG. 12), the first direction X_(I) (323) can be identified based on the front facing direction X at the time of the calibration pose (e.g., illustrated in FIG. 1).

When the head mounted device (111) has a camera (309) as illustrated in FIG. 12, it is not necessary to instruct the user to look straight ahead in the horizontal plane at the calibration pose. For example, the user may look down into the optical marks (312, 314) with or without a rotation along the vertical axis Z. In such a configuration, the first direction X_(I) (323) can be computed from the image (317) of the optical marks (312 and 314) captured by the camera (309) at the time of the calibration pose (e.g., illustrated in FIG. 12), using the relations illustrated in FIG. 13.

FIG. 15 shows a detailed method to calibrate the front facing direction of a head mounted device according to one embodiment. For example, the method of FIG. 15 can be implemented in a system illustrated in FIG. 4, using a calibration pose illustrated in FIG. 5 or 12 relative to a reference pose illustrated in FIG. 1 and a skeleton model of FIG. 10, in accordance with the rotational offset illustrated in FIG. 13.

The method of FIG. 15 includes: capturing (341), using a camera (309) of a head mounted device (111), an image (317) of optical marks (312 and 314) attached on handheld devices (117 and 119) that are in the hands of a user while the user is in a calibration pose (e.g., illustrated in FIG. 12); obtaining (343), from the head mounted device (111), an orientation measurement of the head mounted device (111) at the calibration pose (e.g., illustrated in FIG. 12) relative to a reference pose (e.g., illustrated in FIG. 12); identifying (345), from the image (317), a direction V_(I) (321) of a center O of the optical marks relative to the head mounted device (111); applying (347) an offset co (324) to the direction V_(I) (321) to obtain a first direction X_(I) (323) of the head mounted device (111) at the reference pose (e.g., illustrated in FIG. 12); computing (349) a second direction H (301) of the head mounted device (111) at the calibration pose (e.g., illustrated in FIG. 1) that is rotated by an angle α (305) from the first direction X_(I) (323) according to the orientation measurement; projecting (351) the second direction H (301) to a horizontal plane XY to obtain a third direction H_(h) (303); calculating (353) a rotation (307) from a front facing direction X of the user at the reference pose (e.g., illustrated in FIG. 1) to the third direction H_(h) (303) in the horizontal plane XY; and calibrating (355) subsequent orientation measurements of the head mounted device (111) using the rotation β (307).

The present disclosure includes a system that has a sensor module (111) having an inertial measurement unit (121) and attached to a head (107) of a user, and a computing device (e.g., 141) coupled to the sensor module (111). The computing device (141) may be separate from the sensor module (111), as illustrated in FIG. 4, or be part of the sensor module (111). In some instances, the sensor module (111) is a head mounted display device with a camera (309).

The sensor module (111) generates a measurement of an orientation of the head (107) at a calibration pose (e.g., illustrated in FIG. 5 or 12) relative to a reference pose (e.g., illustrated in FIG. 1). The computing device (141) is configured to: apply a first rotation (e.g., a (305)), defined by the measurement of the orientation of the head at the calibration pose relative to the reference pose, to a first direction (e.g., X or X_(I)) relative to the head at the reference pose to obtain a second direction H (301) relative to the reference pose; project the second direction H (301) on to a horizontal plane XY to obtain a projected direction H_(h) (303) within the horizontal plane XY relative to the reference pose; compute a second rotation β (307) between the projected direction H_(h) (303) and a predetermined direction X of the head (107) relative to the reference pose; and calibrate subsequent orientation measurements of the sensor module (111) using the second rotation β (307).

For example, at the calibration pose as illustrated in FIG. 5 or 12, arms (103 and 105) of the user are in a horizontal plane XY to form predetermined angles with respect to a front facing direction X of a torso of the user at the calibration pose; and at the reference pose as illustrated in FIG. 1, the arms (103 and 105) are in the horizontal plane and are perpendicular to a front facing direction X of the torso of the user at the reference pose. When the calibration is subsequently re-performed, the previously performed calibration pose becomes the reference pose for the subsequent calibration.

The user may be instructed to look straight ahead horizontally; and in such a situation, the first direction (e.g., X or X_(I)) is a front facing direction X at the reference pose.

When the sensor module (111) is a head mounted device having a camera (309), it is not necessary for the user to look straight ahead horizontally at the calibration pose. The first direction X_(I) (323) can be determined from an image (317) captured by the camera (309) at the calibration pose.

For example, the image (317) captures at least one optical mark (e.g., 312, 314) positioned in front of the user at the calibration pose (e.g., illustrated in FIG. 12); and the first direction X_(I) (323) is determined based on a direction V_(I) (321) of the at least one optical mark (e.g., 312, 314) relative to the camera (309) at the calibration pose.

As illustrated in FIG. 13, the computing device may apply a predetermined offset ω (322/324) to the direction V_(I) (321) of the at least one optical mark (e.g. 312, 314) relative to the camera (309) to obtain the first direction X_(I) (323). The predetermined offset ω (322/324) is equal to an offset, at a time when the user looks straight ahead horizontally, between a direction V_(O) (311) of the at least one optical mark relative to the camera (309) and a front facing direction X or x.

Alternatively, as illustrated in FIG. 13, the computing device may compute a rotation offset (326), from a direction V_(I) (321) of the at least one optical mark (312 and 314) relative to the camera (309) at the calibration pose, to a direction V_(O) (311) of the at least one optical mark (312 and 314) relative to the camera (309) when a front facing direction x of the sensor module is in parallel with a front facing direction X of a torso (101) of the user at the calibration pose. Then, the first direction X_(I) (323) can be computed based on the rotation offset (326) by applying the equal rotation offset (325) to the front facing direction x of the sensor module (111).

For example, two handheld devices (117 and 119) having inertial measurement units can be used to measure orientations of hands (106 and 108) of the user. The at least one optical mark (312 and 314) can be configured on the two handheld devices (e.g., 117 and 119) as LED lights, painted marks, structure features, etc. The calibration pose may be detected based on one or more user input generated using the handheld devices (117 and 119), such as button clicks, touch inputs, joystick inputs.

In some instances, two arm modules (113 and 115) having inertial measurement units (e.g., 131) are also used to measure orientations of the arms (103 and 105) of the user. The calibration pose can be detected based on a gesture determined using orientation measurements of the arm modules (103 and 105) and/or the handheld devices (117 and 119).

The sensor module (111) may include a communication device (123) for a communication link with the computing device (141). The inertial measurement unit (121) of the sensor module (111) may include a micro-electromechanical system (MEMS) gyroscope, a magnetometer, and/or a MEMS accelerometer.

The inertial measurement units (e.g., 121, 131) in the sensor modules (e.g., 111 to 119) measure their orientations and/or positions through integration of rate measurements (e.g., acceleration, angular velocity) over a period of time starting from a calibration (e.g., performed using poses illustrated in FIGS. 1-3, FIG. 5, or FIG. 12). Errors in the rate measurements accumulate over time through the integration during the period of time, which can result in an increasing drift of the orientation and/or position measurements from their accurate values. After a threshold period of time, the drift in orientation and/or position measurements of the inertial measurement units (e.g., 121, 131) can be corrected via another calibration operation.

The frequency of the calibrations of the inertial measurement units (e.g., 121, 131) can be reduced by correcting the accumulated errors based on the directions of the optical marks (312 and 314) measured using the camera (309) during the application usage of the sensor modules (e.g., 111 to 119), such as during a session of virtual reality, mixed reality or augmented reality display. The directions of the optical marks (312 and 314) as captured in the images generated by the camera (309) can be compared with the corresponding directions computed from the orientation and/or position measurements of the inertial measurement units (e.g., 121, 131). The orientation and/or position measurements of the inertial measurement units (e.g., 121, 131) can be corrected to remove or reduce the differences between the optical mark directions measured using the camera (309) and the respective directions determined using the inertial measurement units (e.g., 121, 131).

FIG. 16 illustrates the use of a head mounted camera for the direction measurements of optical marks on sensor modules.

In FIG. 16, the user has sensor modules (111 to 119) attached to the head (107), upper arms (103 and 105) and hands (106 and 108), in a way similar to the configuration illustrated in FIG. 1. At least some of the sensor modules (e.g., 117, 119, 113, and/or 115) have optical marks (e.g., 312, 314) in a way as illustrated in and discussed above in connection with FIG. 12.

After the sensor modules (111 to 119) are calibrated through the use of poses of FIGS. 1 to 3 and/or subsequently recalibrated using the pose of FIG. 5 or 12, the sensor modules (111 to 119) may have accumulated errors through integration over a period of time when some of the optical marks are detectable in the image generated by the camera (309) of the head module (111). As a result of the accumulated errors, the directions and/or the positions of the optical marks (312, 314) on the hand modules (117 and 119) as determined from the image may be different from the directions and/or the positions of the optical marks (312, 314) (e.g., as illustrated in FIG. 18) tracked via the inertial measurement units (e.g., 121, 131) of the sensor modules (111 to 119).

In FIG. 16, the optical marks (312 and 314) are in the field of view (319) of the camera (309) of the head module (111). Using the image(s) generated by the camera (309) and the orientation of the head module (111), the computing device (141) calculates one or more directions of the optical marks (312, 314) relative to the user in a global coordinate system (100). Separately, the computing device (141) calculates the corresponding one or more directions of the optical marks (312, 314) relative to the user in the global coordinate system (100) according to the orientations and/or positions of the arm modules (113 and 115) and/or the hand modules (117 and 119). The differences between the optical mark directions computed from the image generated by the camera (309) and from the sensor modules (113, 115, 117 and/or 119) are indications of accumulated errors. The orientations and/or positions of the head module (111), the arm modules (113 and 115) and/or the hand modules (117 and 119) can be adjusted to reduce or eliminate the differences.

FIG. 16 illustrates a scenario where at least some of the optical marks (e.g., 312 and 314) are within the field of view (319) of the camera (309), which allows the computing device (141) to reduce or eliminate the accumulated errors in the inertial measurement units (e.g., 121, 131) in the sensor modules (e.g., 111 to 119) using the images generated by the camera (309). However, when the optical marks (e.g., 312 and 314) are moved out of the field of view (319) (e.g., as illustrated in FIG. 17), the errors in the inertial measurement units (e.g., 121, 131) will accumulate over time.

FIG. 17 illustrates a scenario where the direction measurements of optical marks on sensor modules cannot be determined using a head mounted camera (309). If the optical marks (e.g., 312 and 314) are out of the field of view for a period of time that is longer than a threshold, the accumulated errors may be sufficiently large; and the user may be prompted to enter a calibration pose as illustrated in FIG. 5 or 12, or recalibration via poses illustrated in FIGS. 1 to 3, or move at least some of the optical marks (e.g., 312 and 314) into the field of view (319) (e.g., as illustrated in FIG. 16).

FIGS. 16 and 17 illustrate a configuration where the computing device (141) is separate from the sensor modules (111 to 119). In other configurations, the computing device (141) is integrated within the head module (111), or one of the hand modules (117 and 119).

Optionally, the camera (309) implements a stereoscopic vision for the determination of a position of an optical mark (e.g., 312 or 314) in the field of view (319), which allows the determination of not only the direction of the optical mark (e.g., 312 or 314) relative to the camera (309) but also the distance of the optical mark (e.g., 312 or 314) to the camera (309). Based such the measurement of the direction and distance of the optical mark (e.g., 312 or 314), the direction of the optical mark (e.g., 312 or 314) relative to other parts of the user, such as the shoulder joints (102 and 104) of the user, can also be calculated based on a skeleton model of the user. When the camera (309) does not have a stereoscopic vision; and the direction of optical mark (e.g., 312 or 314) relative to the camera (309) is used directly or indirectly in the reduction or elimination of accumulated errors, as further discussed below.

FIG. 18 illustrates a skeleton model for the correction of accumulated errors in inertial measurement units according to one embodiment.

The skeleton model of FIG. 18 has the positions of the arms (103 and 105) and the hand modules (117 and 119) tracked using the orientation measurements of at least the arm models (113 and 115). FIG. 18 also shows the positions (367 and 369) of the hand modules (117 and 119) as observed by the camera (309).

When the camera (309) has a stereoscopic vision, the camera (309) determines not only the direction from the camera (309) to the position (367) of the hand module, as indicated by the optical mark (314) observed by the camera (309), but also the depth of the position (367) of the hand module (117) from the camera (309). The combination of the direction and depth provides the location of the position (367) in the space, which can be subsequently used to compute the direction T_(R) (361) from the shoulder joint (104) of the user. The arm module (115) provides the lengthwise direction V_(R)(371) of the right arm (105). The rotation θ_(R) (365) from the direction V_(R) (371), as measured by the arm module (115), to the direction T_(R) (361), as measured using the camera (309), represents an accumulated error in the orientation measurement of the arm module (115). Therefore, the rotation θ_(R) (365) can be applied to the orientation measurement of the arm module (115) to remove or reduce the accumulated error.

Similarly, FIG. 18 illustrates the lengthwise direction V_(L) (373) of the left arm (103), as measured by the arm module (113), and the direction T_(L) (363) of the hand module (119), as measured using the camera (309). The rotation θ_(L) (366) between the direction V_(L) (373) and the direction T_(L) (363) can be applied to the orientation measurement of the arm module (113) to remove or reduce the accumulated error.

In some implementations, the common mode of the rotations θ_(L) (366) and θ_(R) (365) are considered the accumulated error of the orientation of the head module (111) to which the camera (309) is attached; and the differences between the common mode and the rotations θ_(L) (366) and θ_(R) (365) respectively are used to correct the orientation measurements of the arms (103 and 105). For example, a common mode of rotation of (θ_(L)−θ_(R))/2 can be applied to the orientation measurement of the head module (111); a rotation of θ_(L)−(θ_(L)−θ_(R))/2 is applied to the orientation measurement of the left arm module (113); and a rotation of θ_(R)−(θ_(R)−θ_(L))/2 is applied to the orientation measurement of the right arm module (115).

When the camera (309) does not have a stereoscopic vision, the position (367) of the hand module (117) can be determined from the intersection between the line of sight of the hand module (117) as observed by the camera (309) and the circle (spherical surface) that is centered at the shoulder joint (104) and has a radius equal to the distance between the hand module (117) and the shoulder joint (104). Once the position (367) of the hand module (117) is determined in the space, the direction T_(R)(361) pointing from the shoulder joint (104) to the position (367) can be calculated for the removal or reduction of the accumulated errors.

After the orientations of the arm modules (113 and 115) are corrected according to the observed positions (367 and 369) of the hand modules (117 and 119), as indicated by their optical markers (312 and 314) observed by the camera (309), the skeleton model of FIG. 18 is updated to a state as illustrated in FIG. 19.

FIG. 19 illustrates corrections made according to the model of FIG. 18. In FIG. 19, the positions of the hand modules (117 and 119) as determined from the orientation measurements of the arm modules (113 and 115) coincide with their positions observed by the camera (309).

In some instances, the skeleton model of FIG. 18 or 19 is used to control a computer display, such as an avatar of the user in virtual reality, or a display of augmented reality or mixed reality. An instantaneous correction from the skeleton model of FIG. 18 to the skeleton model of FIG. 19 may create undesirable artifacts in the display and/or uncomfortable sensation in user experience, such as jump like movement of the hand of the avatar, a sudden change of a view selected according to the skeleton model of the user. To reduce the undesirable artifacts and/or uncomfortable sensations, the correction can be applied over a period of time with increments, where each increment does not exceed a threshold. For example, each incremental correction is limited to move the position of the hand module (117) by no more than a threshold (e.g., 5 mm); and each successive corrections are separated by a time gap to smooth the corrections over a period of time.

During a time period in which the optical marks (312 and 314) are within the field of view (319) of the camera (309) (e.g., as illustrated in FIG. 17), the computation of the rotation θ_(R) (365) that is representative of the accumulated error of the arm module (115) can be performs as illustrated in FIG. 18 and repeated periodically to perform the error reduction illustrated in FIG. 19. The time period to repeat the calculation can be selected to balance the computation cost and the magnitude of the accumulated error. Preferably, the correction is computed during time periods where the optical marks (312 and 314) are moving in the field of view (319) no faster than a threshold speed and the positions/directions of the optical marks (312 and 314) can be accurately determined from the images of the camera (309).

FIGS. 18 and 19 illustrate an example where the left and right arms (103 and 105) remain straight. When the left and right arms (103 and 105) bend at respective elbow joints, the direction (e.g., V_(L) (373)) from the shoulder joint (e.g., 102) to the hand module (e.g., 119) may not be the same as the lengthwise direction of the upper arm (105) as determined by the arm module (e.g., 113). In general, the direction (e.g., V_(L) (373)) from the shoulder joint (e.g., 102) to the hand module (e.g., 119) or the hand (e.g., 108) is calculated from the orientations of the upper arm (e.g., 103), the hand (e.g., 108) and/or the forearm connecting the upper arm (e.g., 103) and the hand (e.g., 108).

Optionally, a further arm module can be attached to the forearm and used to track the orientation of the forearm. Alternative, the orientation of the forearm can be estimated and/or computed from the orientation of the hand (e.g., 108), as measured by the hand module (e.g., 119), and the orientation of the upper arm (e.g., 103), as measured by the arm module (e.g., 113), as discussed below in connection with FIGS. 20 and 21.

FIGS. 20 and 21 illustrate the computation of the orientation of a forearm (383) based on the orientations of a hand (108) and an upper arm (103) connected by the forearm (383).

FIG. 20 illustrates a skeleton model of an arm having an elbow joint (381) and a wrist joint (385). For example, the skeleton model of FIG. 20 can be used in the motion processor (145) of FIG. 4 to determine the orientation of the forearm (383) that does not have an attached sensor module, as illustrated in FIGS. 1-3, 5, 12, 16, and 17.

FIG. 20 shows the geometrical representations of the upper arm (103), the forearm (383), and the hand (108) in relation with the elbow joint (381) and the wrist (385) relative to the shoulder (102).

Each of the upper arm (103), the forearm (383), and the hand (108) has an orientation relative to a common reference system (e.g., the shoulder (102), a room, or a location on the Earth where the user is positioned). The orientation of the upper arm (103), the forearm (383), or the hand (108) is indicated by a local coordinate system (391, 393, or 395) aligned with the upper arm (103), the forearm (383), or the hand (108) respectively.

The orientation of the upper arm (103) and the orientation of the hand (108), as represented by the local coordinate systems (391 and 395) can be calculated from the motion parameters measured by the IMUs in the module (113 and 119) attached to the upper arm (103) and the hand (108).

Since the forearm (383) does not have an attached IMU for the measurement of its orientation, the motion processor (145) uses a set of assumed relations between the movements of the forearm (383) and the hand (108) to calculate or estimate the orientation of the forearm (383) based on the orientation of the upper arm (103) and the orientation of the hand (108), as further discussed below.

FIG. 21 illustrates the determination of the orientation of a forearm according to one embodiment.

In FIG. 21, the coordinate system X₁Y₁Z₁ represents the orientation of the upper arm (103), where the direction Y₁ is along the lengthwise direction of the upper arm (103) pointing from the shoulder (102) to the elbow joint (381), as illustrated in FIG. 20. The directions X₁ and Z₁ are perpendicular to the direction Y₁. The direction X₁ is parallel to the direction from the back side of the upper arm (103) to the front side of the upper arm (103); and the direction X₁ is parallel to the direction from the inner side of the upper arm (103) to the outer side of the upper arm (103).

When the arm is in a vertical direction pointing downwards with the hand (108) facing the body of the user (e.g., as illustrated in FIG. 2), the lengthwise directions Y₁, Y₂, and Y₃ of the upper arm (103), the forearm (383), and the hand (108) are aligned with the vertical direction pointing downwards. When in such a position, the inner sides of the forearm (383) and the upper arm (103) are closest to the body of the user; and the outer sides of the forearm (383) and the upper arm (103) are away from the body of the user; the directions Z₁, Z₂, and Z₃ of the upper arm (103), the forearm (383), and the hand (108) are aligned with a direction pointing sideway to the user; and the directions Z₁, Z₂, and Z₃ of the upper arm (103), the forearm (383), and the hand (108) are aligned with a direction pointing to the front of the user.

Thus, the plane X₁Y₁ is parallel to the direction X₁ from the back side of the upper arm (103) to the front side of the upper arm (103), parallel to the lengthwise direction Y₁ of the upper arm (103), and perpendicular to the direction Z₁ from the direction from the inner side of the upper arm (103) to the outer side if the upper arm (103). The direction Z₁ coincides with an axis of the elbow joint about which the forearm (383) can rotate to form an angle with the upper arm (103) between their lengthwise directions. When the upper arm (103) is extended in the sideway of the user and in a horizontal position, the directions X₁ and Z₁ are aligned with (in parallel with) the front direction and vertical direction respectively.

The direction Y₂ is aligned with the lengthwise direction of the forearm (383) pointing from the elbow joint (381) to the wrist (385).

The direction Y₃ is aligned with the lengthwise direction of the hand (108) pointing from the wrist (385) towards the fingers.

When the upper arm (103) is extended in the sideway of the user and in a horizontal position, the directions Y₁, Y₂, and Y₃ coincide with the horizontal direction pointing the sideway of the user.

When the hand (108) is moved to an orientation illustrated in FIG. 21, the hand (108) can be considered to have moved from the orientation of the coordinate system X₁Y₁Z₁ through rotating (405) by an angle γ along the lengthwise direction Y₁ and then rotating (401) along the shortest arc (401) such that its lengthwise direction Y₃ arrives at the direction illustrated in FIG. 21. The rotation (401) along the shortest arc (401) corresponding to a rotation of the direction Y₁ by an angle β in a plane containing both the directions Y₁ and Y₃ along an axis perpendicular to both the directions Y₁ and Y₃ (i.e., the axis is perpendicular to the plane containing both the directions Y₁ and Y₃).

The projection of the direction Y₃ in the plane X₁Y₁ is assumed to be in the direction of the lengthwise direction Y₂ of the forearm (383). The projection represents a rotation (403) of the direction Y₁ by an angle α in the plane X₁Y₁ along the direction Z₁ according to the shortest arc (403).

It is assumed that the rotation (405) of the hand (108) along its lengthwise direction is a result of the same rotation of the forearm (383) along its lengthwise direction while the forearm (383) is initially at the orientation aligned with the coordinate system X₁Y₁Z₁. Thus, when the hand (108) has an orientation illustrated in FIG. 21 relative to the orientation (X₁Y₁Z₁) of the upper arm (103), the orientation of the forearm (383) is assumed to have moved from the orientation of the coordinate system X₁Y₁Z₁ by rotating (405) along the lengthwise direction Y₁ and then rotating (403) in the plane Y₁ along the direction Z₁.

Since the rotations (405, 401 and 403) can be calculated from the orientation of the hand (108) relative to the orientation of the upper arm (103) (e.g., using the orientation data measured by the IMUs of the arm module (113) and the handheld module (115)), the orientation of the forearm (383) can be calculated from the rotations (405 and 403) without measurement data from an IMU attached to the forearm (383).

After the orientations of the upper arm (103), the forearm (383) and the hand (108) are obtained, the motion processor (145) can compute the positions of the upper arm (103), the forearm (383) and the hand (108) in a three dimensional space (relative to the shoulder (102)), which allows the application (147) to present an arm of an avatar in a virtual reality, augmented reality, or mixed reality in accordance with the movement of the arm of the user. The positions of the upper arm (103), the forearm (383) and the hand (108) in a three dimensional space (relative to the shoulder (102)) can also be used to determine the gesture made by the user in the three dimensional space to control the application (147) and the direction from the shoulder joint (102) to the hand (108).

Further details and examples of determining/estimating forearm orientations can be found in U.S. patent application Ser. No. 15/787,555, filed Oct. 18, 2017 and entitled “Tracking Arm Movements to Generate Inputs for Computer Systems,” the entire disclosure of which is hereby incorporated herein by reference.

After the orientations of the upper arm (103), the forearm (383) and the hand (108) are computed from the orientation measurements of the arm module (113) and the hand module (119), the skeleton model of the arm of FIG. 20 can be used to compute the straight line direction V_(L) (373) from the shoulder joint (102) to the hand module (119) held in the hand (108) of the user, as well as the straight line distance between the shoulder joint (102) and the hand module (119). Thus, the accumulated error (e.g., θ_(L) (366)) can be identified in a way as illustrated in FIG. 18.

When an optical mark configured on an upper arm (103) is in the field of view (319) of the camera (309), the direction of the optical mark relative to the shoulder (102) as observed by the camera (309) can be used to correct the orientation of the measurement of the upper arm (103) by the corresponding arm module (113); and when optical marks configured on the upper arm (103) and the hand (108) are in the field of view (319) of the camera (309), the orientation of the measurement of the hand (103) can be corrected after the correction of the orientation of the measurement of the upper arm (103).

FIG. 22 illustrates a method to correct accumulated errors in an inertial measurement unit according to one embodiment.

For example, the method of FIG. 22 can be implemented in the system of FIG. 4 using sensor modules (111 to 119) attached to a user in a way illustrated in FIG. 1, the skeleton model of FIG. 18, during a time period in which the optical marks (312 and 314) are within the field of view (319) of the camera (309) illustrated in FIG. 16, after the sensor modules (111 to 119) are calibrated according to the poses of FIGS. 1 to 3, and/or recalibrated using the pose of FIG. 5 or 12.

The method of FIG. 22 includes: receiving (421) an measurement of the orientation of a sensor module (e.g., 113 or 115) attached to an arm (e.g., 103 or 105) of a user; measuring (423) a direction (e.g., T_(L) (363) or T_(R) (361)) of an optical mark (e.g., 312 or 314) using a head mounted device (e.g., 111) having a camera (e.g., 309); determining (425) a difference (e.g., θ_(L) (366) or θ_(R) (365)) in the direction (e.g., T_(L) (363) or T_(R) (361)) of the optical mark (e.g., 312 or 314) measured using the head mounted device (e.g., 111) and a direction (e.g., V_(L) (373) or V_(R) (371)) of the optical mark (e.g., 312 or 314) determined using the measurement from the sensor module (e.g., 113 or 115); and computing (427) a correction to the orientation of the sensor module based on the difference (e.g., θ_(L) (366) or θ_(R) (365)).

For example, a system to implement the method of FIG. 22 may include: a plurality of sensor modules (111 to 119) having inertial measurement units (e.g., 121 and 131) and attached to different parts of a user (e.g., head (107), hands (106 and 108), arms (105 and 103)) to measure their orientations; a plurality of optical marks (e.g., 312 and 314) attached to the user; a camera (309) attached to the user; and a computing device (141) configured to correct an accumulated error in the measurement of an inertial measurement units (e.g., 121 or 131) by detecting the optical marks (e.g., 312 and 314) in an image generated by the camera (309) and identifying mismatches (e.g., θ_(L) (366) and θ_(R) (365)) in directions (e.g., T_(L) (363) and T_(R) (361)) of the optical marks (312 and 314) (or the sensors (e.g., 117 and 119), or parts (e.g., 106 and 108) of the users) as measured and/or calculated based on the image generated by the camera (309) and the corresponding directions (e.g., V_(L) (373) and V_(R) (371)) of the optical marks (312 and 314) (or the sensors (e.g., 117 and 119), or parts (e.g., 106 and 108) of the users) as measured and/or calculated from the orientation measurements generated by the sensor modules (113, 115, 117 and/or 119).

For example, the computing device (141) is configured via computer instructions to: receive orientation measurements from the sensor modules (111 to 119); receive an image captured by the camera (309); and detect the plurality of optical marks (312 and 314) in the image captured by the camera (309) to identify an accumulated error of at least one of the received orientation measurements based on the image and the orientation measurements. The at least one of the orientation measurements is corrected at least in part by removing at least a portion of the accumulated error.

For example, the accumulated error is divided into a plurality of portions for corrections made over a period of time. Subsequent corrections are separated by a time interval such that the correction of the accumulated error is smoothed out over the period of time to avoid an instantaneous correction of a significant gap. For example, each of the corrections for a corresponding one of portions is selected such that the correction moves a portion of the skeleton model of the user in space by less than a threshold distance (e.g., 5 mm).

For example, the camera (309) is attached to or integrated in a head mounted display that may also include the computing device (141) and a head sensor module (111) that determines the orientation of the head (107)/camera (309) of the user. The directions (e.g., T_(L) (363) and T_(R) (361)) of the optical marks (312 and 314) (or the sensors (e.g., 117 and 119), or parts (e.g., 106 and 108) of the users) are as measured and/or calculated based on the image generated by the camera (309) and the orientation measurement of the head module (111).

For example, at least some of the optical marks (312 and 314) are configured on the hand modules (117 and 119) such that the positions of the optical marks (312 and 314) identify the positions of the hand modules (117 and 119) and/or the positions of the hands (106 and 108) of the user. Alternatively, the optical marks (312 and 314) may be separately configured on the hands (106 and 108) of the user without being on the hand modules (117 and 119). Further, some of the optical marks in the system are configured on the arm modules (113 and 115). Alternatively, or in combination, some of the optical marks in the system are attached to the user separately from the sensor modules (e.g., 111 to 119). For example, an armband or wristband having one or more optical marks can be worn on the wrist (385) of the user or the forearm (383) without a sensor module.

In some configurations of the system, the orientation of the forearm (383) is estimated or calculated from the orientation of the upper arm (103) and the hand (108), without a sensor module being attached to the forearm (383). Alternatively, the orientation of the forearm (383) can be measured using an additional sensor module attached to the forearm (383).

For example, when an optical mark (314) indicative of the position of the hand (106) of the user is in the image captured by the camera (309), the direction T_(R) (361) of the optical mark (314) (or the hand (106) or the hand module (117)) is calculated from the line of sight direction of the optical mark (314) as indicated in the image. The corresponding direction V_(R) (371) is measured and/or calculated based on the orientation measurements of the arm module (115) and/or the hand module (117). A rotation θ_(R) (365) between the direction T_(R) (361) and the corresponding direction V_(R) (371) identifies an accumulated error, which can be corrected by rotating the orientation measurements of the arm module (115) and/or the hand module (117) to close the gap.

For example, when both optical marks (312, 314) indicative of the positions of the hands (106 and 108) of the user are in the image captured by the camera (309), the direction T_(R) (361) of the optical mark (314) and the direction T_(L) (363) of the optical mark (312) are calculated respectively from the line of sight directions of the optical marks (312 and 314) as indicated in the image. The corresponding directions V_(R) (371) and V_(L) (373) are measured and/or calculated based on the orientation measurements of the arm modules (115 and 113) and/or the hand modules (117 and 119). The rotation θ_(R) (365) between the direction T_(R) (361) and the corresponding direction V_(R) (371) and the rotation θ_(L) (366) between the direction T_(L) (363) and the corresponding direction V_(L) (373) identify accumulated errors, which can be corrected by rotating the orientation measurements of the arm modules (115 and 113), the hand modules (117 and 119), and/or the head module (111) to close the gaps (θ_(L) (366) and θ_(R) (365)).

For example, the orientation measurements of the left arm module (113) and/or left hand module (119) can be corrected according to the rotation θ_(L) (366) without considering the rotation θ_(R) (365); and the orientation measurements of the right arm module (115) and/or right hand module (117) can be corrected according to the rotation θ_(R) (365) without considering the rotation θ_(L) (366).

Alternative, the orientation measurement of the head module (111) is corrected based on both rotations θ_(R) (365) and θ_(L) (366) (e.g., a common mode of θ_(R) (365) and θ_(L) (366)). In such a configuration, the orientation measurements of the left arm module (113) and/or left hand module (119) can be corrected according to the rotation θ_(L) (366), in view of the rotation θ_(R) (365) (e.g., correcting according to the difference between the rotation θ_(L) (366) and the common mode); and the orientation measurements of the right arm module (115) and/or right hand module (117) can be corrected according to the rotation θ_(R) (365), in view of the rotation θ_(L) (366) (e.g., correcting according to the difference between the rotation θ_(R) (365) and the common mode).

The present disclosure includes methods and apparatuses which perform these methods, including data processing systems which perform these methods, and computer readable media containing instructions which when executed on data processing systems cause the systems to perform these methods.

For example, the computing device (141), the handheld devices (117 and 119), the arm modules (113, 115), and/or the head module (111) can be implemented using one or more data processing systems.

A typical data processing system may include includes an inter-connect (e.g., bus and system core logic), which interconnects a microprocessor(s) and memory. The microprocessor is typically coupled to cache memory.

The inter-connect interconnects the microprocessor(s) and the memory together and also interconnects them to input/output (I/O) device(s) via I/O controller(s). I/O devices may include a display device and/or peripheral devices, such as mice, keyboards, modems, network interfaces, printers, scanners, video cameras and other devices known in the art. In one embodiment, when the data processing system is a server system, some of the I/O devices, such as printers, scanners, mice, and/or keyboards, are optional.

The inter-connect can include one or more buses connected to one another through various bridges, controllers and/or adapters. In one embodiment the I/O controllers include a USB (Universal Serial Bus) adapter for controlling USB peripherals, and/or an IEEE-1394 bus adapter for controlling IEEE-1394 peripherals.

The memory may include one or more of: ROM (Read Only Memory), volatile RAM (Random Access Memory), and non-volatile memory, such as hard drive, flash memory, etc.

Volatile RAM is typically implemented as dynamic RAM (DRAM) which requires power continually in order to refresh or maintain the data in the memory. Non-volatile memory is typically a magnetic hard drive, a magnetic optical drive, an optical drive (e.g., a DVD RAM), or other type of memory system which maintains data even after power is removed from the system. The non-volatile memory may also be a random access memory.

The non-volatile memory can be a local device coupled directly to the rest of the components in the data processing system. A non-volatile memory that is remote from the system, such as a network storage device coupled to the data processing system through a network interface such as a modem or Ethernet interface, can also be used.

In the present disclosure, some functions and operations are described as being performed by or caused by software code to simplify description. However, such expressions are also used to specify that the functions result from execution of the code/instructions by a processor, such as a microprocessor.

Alternatively, or in combination, the functions and operations as described here can be implemented using special purpose circuitry, with or without software instructions, such as using Application-Specific Integrated Circuit (ASIC) or Field-Programmable Gate Array (FPGA). Embodiments can be implemented using hardwired circuitry without software instructions, or in combination with software instructions. Thus, the techniques are limited neither to any specific combination of hardware circuitry and software, nor to any particular source for the instructions executed by the data processing system.

While one embodiment can be implemented in fully functioning computers and computer systems, various embodiments are capable of being distributed as a computing product in a variety of forms and are capable of being applied regardless of the particular type of machine or computer-readable media used to actually effect the distribution.

At least some aspects disclosed can be embodied, at least in part, in software. That is, the techniques may be carried out in a computer system or other data processing system in response to its processor, such as a microprocessor, executing sequences of instructions contained in a memory, such as ROM, volatile RAM, non-volatile memory, cache or a remote storage device.

Routines executed to implement the embodiments may be implemented as part of an operating system or a specific application, component, program, object, module or sequence of instructions referred to as “computer programs.” The computer programs typically include one or more instructions set at various times in various memory and storage devices in a computer, and that, when read and executed by one or more processors in a computer, cause the computer to perform operations necessary to execute elements involving the various aspects.

A machine readable medium can be used to store software and data which when executed by a data processing system causes the system to perform various methods. The executable software and data may be stored in various places including for example ROM, volatile RAM, non-volatile memory and/or cache. Portions of this software and/or data may be stored in any one of these storage devices. Further, the data and instructions can be obtained from centralized servers or peer to peer networks. Different portions of the data and instructions can be obtained from different centralized servers and/or peer to peer networks at different times and in different communication sessions or in a same communication session. The data and instructions can be obtained in entirety prior to the execution of the applications. Alternatively, portions of the data and instructions can be obtained dynamically, just in time, when needed for execution. Thus, it is not required that the data and instructions be on a machine readable medium in entirety at a particular instance of time.

Examples of computer-readable media include but are not limited to non-transitory, recordable and non-recordable type media such as volatile and non-volatile memory devices, read only memory (ROM), random access memory (RAM), flash memory devices, floppy and other removable disks, magnetic disk storage media, optical storage media (e.g., Compact Disk Read-Only Memory (CD ROM), Digital Versatile Disks (DVDs), etc.), among others. The computer-readable media may store the instructions.

The instructions may also be embodied in digital and analog communication links for electrical, optical, acoustical or other forms of propagated signals, such as carrier waves, infrared signals, digital signals, etc. However, propagated signals, such as carrier waves, infrared signals, digital signals, etc. are not tangible machine readable medium and are not configured to store instructions.

In general, a machine readable medium includes any mechanism that provides (i.e., stores and/or transmits) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.).

In various embodiments, hardwired circuitry may be used in combination with software instructions to implement the techniques. Thus, the techniques are neither limited to any specific combination of hardware circuitry and software nor to any particular source for the instructions executed by the data processing system.

In the foregoing specification, the disclosure has been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense. 

What is claimed is:
 1. A system, comprising: a plurality of sensor modules, each having an inertial measurement unit and attached to a portion of the user to generate motion data identifying an orientation of the portion of the user; a plurality of optical marks attached to the user; a camera attached to the user; a computing device configured to: receive orientation measurements from the plurality of sensor modules; receive an image captured by the camera; detect the plurality of optical marks in the image captured by the camera; identify an accumulated error of at least one of the orientation measurements based on the detected plurality of optical marks in the image and the orientation measurements; and correct the at least one of the orientation measurements by removing at least a portion of the accumulated error.
 2. The system of claim 1, wherein the plurality of optical marks includes a first optical mark; and the computing device is further configured to: calculate a first direction of the first optical mark relative to the user based on the detected plurality of optical marks in the image; calculate a second direction of the first optical mark relative to the user based on a skeleton model of the user and the orientation measurements received from the plurality of sensor modules; compute a rotation between the first direction and the second direction to identify the accumulated error; and removing the at least the portion of the accumulated error by rotating the orientation measurements of at least one of the plurality of sensor modules.
 3. The system of claim 2, wherein the camera is attached to a head of the user.
 4. The system of claim 3, wherein the camera is attached to the head of the user via a first sensor module in the plurality of sensor modules; and the first direction is determined further based on an orientation measurement of the first sensor.
 5. The system of claim 4, wherein a second sensor module is in the plurality of sensor modules attached to an upper arm of the user; wherein the second direction is determined based on an orientation measurement of the second sensor.
 6. The system of claim 5, wherein the first optical mark is configured on a third sensor module in the plurality of sensor modules, the third sensor module attached to a hand of the user.
 7. The system of claim 6, wherein the computing device is further configured to: calculate an orientation of a forearm connected between the hand and the upper arm based on the orientation measurement of the second sensor attached to the upper arm and the orientation measurement of the third sensor attached to the hand, wherein the second direction is computed based on the orientation of the forearm.
 8. The system of claim 7, wherein the orientation of the forearm is calculated without a sensor module attached to the forearm.
 9. The system of claim 1, wherein the computing device is further configured to: divide the accumulated error into a plurality of portions; and perform a plurality of corrections for the plurality of portions respectively over a period of time.
 10. The system of claim 9, wherein each of the plurality of corrections is selected to move a skeleton model of the user less than a threshold distance.
 11. The system of claim 1, wherein the plurality of optical marks include a first optical mark attached to a left hand of the user and a second optical mark attached to a right hand of the user; and the computing device is further configured to: calculate a first rotation from a direction of the left hand determined from the first optical mark in the image and the direction of the left hand determined from the orientation measurements; calculate a second rotation from a direction of the right hand determined from the second optical mark in the image and the direction of the right hand determined from the orientation measurements; and determine an accumulated error in an orientation measurement of a sensor module on which the camera is installed based on the first rotation and the second rotation.
 12. The system of claim 11, wherein the sensor module, the camera and the computing device are configured on a head mounted display device.
 13. The system of claim 11, wherein the direction of the left hand is relative to a left shoulder joint of the user; and the direction of the right hand is relative to the right shoulder joint of the user.
 14. The system of claim 11, wherein the plurality of sensor modules includes a first sensor module attached to a left upper arm of the user and a second sensor module attached to a right upper arm of the user; and the computing device is further configured to: determine an accumulated error in an orientation measurement of the first sensor module based at least in part on the first rotation; and determine an accumulated error in an orientation measurement of the second sensor module based at least in part on the second rotation.
 15. The system of claim 11, wherein the accumulated error in the orientation measurement of a first sensor module is determined further based on the second rotation; and the accumulated error in the orientation measurement of the second sensor module is determined further based on the first rotation.
 16. The system of claim 1, wherein each of the plurality of sensor modules further includes a communication device for a communication link with the computing device.
 17. The system of claim 1, wherein the inertial measurement unit includes a micro-electromechanical system (MEMS) gyroscope.
 18. The system of claim 17, wherein the inertial measurement unit further includes a magnetometer and the MEMS accelerometer.
 19. A method, comprising: receiving, in a computing device, orientation measurements from a plurality of sensor modules, each of the plurality of sensor modules having an inertial measurement unit and attached to a portion of the user to identify an orientation of the portion of the user; receiving, in the computing device, an image captured by a camera attached to the user; detecting, by the computing device, a plurality of optical marks in the image captured by the camera, the optical marks attached to the user; identifying an accumulated error of at least one of the orientation measurements based on the detected plurality of optical marks in the image and the orientation measurements; and correcting the at least one of the orientation measurements by removing at least a portion of the accumulated error.
 20. A non-transitory computer storage medium storing instructions which, when executed by a computing device, instructs the computing device to perform a method, the method comprising: receiving, in the computing device, orientation measurements from a plurality of sensor modules, each of the plurality of sensor modules having an inertial measurement unit and attached to a portion of the user to identify an orientation of the portion of the user; receiving, in the computing device, an image captured by a camera attached to the user; detecting, by the computing device, a plurality of optical marks in the image captured by the camera, the optical marks attached to the user; identifying an accumulated error of at least one of the orientation measurements based on the detected plurality of optical marks in the image and the orientation measurements; and correcting the at least one of the orientation measurements by removing at least a portion of the accumulated error.
 21. The system of claim 1, wherein the computing device is further configured to determine a difference in a first direction of at least one optical mark of the plurality of optical marks detected in the image and a second direction of the at least one optical mark based on the orientation measurements and identify the accumulated error of at least one of the orientation measurements based on the difference of the first direction and the second direction. 